THIS IS A SHANNON AWARD PROVIDING PARTIAL SUPPORT FOR THE RESEARCH PROJECTS THAT FALL SHORT OF THE ASSIGNED INSTITUTE'S FUNDING RANGE BUT ARE IN THE MARGIN OF EXCELLENCE. THE SHANNON AWARD IS INTENDED TO PROVIDE SUPPORT TO TEST THE FEASIBILITY OF THE APPROACH; DEVELOP FURTHER TESTS AND REFINE RESEARCH TECHNIQUES; PERFORM SECONDARY ANALYSIS OF AVAILABLE DATA SETS; OR CONDUCT DISCRETE PROJECTS THAT CAN DEMONSTRATE THE PI'S RESEARCH CAPABILITIES OR LEAD ADDITIONAL WEIGHT TO AN ALREADY MERITORIOUS APPLICATION. THE APPLICATION BELOW IS TAKEN FROM THE ORIGINAL DOCUMENT SUBMITTED BY THE PRINCIPAL INVESTIGATOR. Exercise has a number of positive cardiovascular health benefits. However, exercise-related injuries are very common and often a deterrent to resumption of exercise. Stress fractures are extremely common among amateur, high school, collegiate, and professional athletes. Incidence rates vary, but it appears that the rate of stress fractures among high school athletes are between 4-8% per year. While good surveillance data are not available, rates of stress fractures appear to be particularly high among runners, soccer players, and basketball players. These injuries are costly, painful, and can be career threatening. While prevention is an obvious goal, little is know about potential methods for reducing the prevalence of stress fractures. Low bone density and low dietary calcium levels are known to be risk factors for stress fractures. Additionally, recently collected data in our laboratory, as well as others, suggest that some athletes are at risk for substantial drops in bone density over time. Intervention with calcium supplementation coincided with marked increases in bone density. Clearly, these results merit replication in a larger, more heterogeneous cohort. Moreover, data needs to be collected to determine if calcium supplementation is feasible with large groups of athletes. Thus, the Specific Aims of this project are as follows: 1. To replicate and extend our findings that suggest that certain athletes (in this proposal, high school basketball players) are at risk for decreased bone density by assessing both male and female high school basketball players over the course of a season (Years 1 + 2); 2. To assess potential mechanisms for these bone density changes (e.g., estrogen, testosterone, calcium intake, markers of bone turnover, Years 1 + 2); and 3. To conduct a small-scale intervention on a cohort of athletes to determine intervention feasibility (i.e., can changes occur in the Rx group and not the control), safety (side effects, calcium/creatinine, excessively high calcium intakes) and short-term intervention effects (change in calcium intake over time, change in bone density over the course of a season; Year 3).